Risk factors of bloodstream infection after allogeneic hematopoietic cell transplantation in children/adolescent and young adults

Allogeneic hematopoietic cell transplantation (HCT) is a crucial treatment for various diseases, including hematological malignancies, solid tumors, and genetic disorders. Despite its curative potential, HCT is associated with severe complications, notably infections, graft-versus-host disease, and organ damage. Infections, particularly bloodstream infections (BSIs), pose a significant threat in the initial weeks post-HCT, necessitating effective management strategies. This retrospective study aimed to clarify the incidence, pathogens, and risk factors associated with BSI within the first 30 days after allogeneic HCT in children/adolescents and young adults (AYAs). The study included 115 patients aged <31 years who underwent 121 allogeneic HCTs at the Department of Pediatrics, Nagoya University Hospital between January 1, 2018, and March 31, 2022. Data encompassed demographic characteristics, HCT details, and BSI information. Overall, 27 of 121 patients developed BSI with the cumulative incidence of 23.5% (95% confidence intervals [CI]: 17.0%–30.6%) at 30 days after HCT. The median onset time of BSI was 7 (range, 4–26 days) after HCT. Gram-positive bacteria accounted for 89% of pathogens isolated from blood cultures, with Streptococcus mitis/oralis being the most common. In multivariable analysis, tandem HCT (subdistribution hazard ratio [SHR]: 5.67, 95% CI: 2.74–11.7, p < 0.001) and peripherally inserted central catheters (SHR: 2.96, 95% CI: 1.34–6.55, p  =  0.007) were identified as independent risk factors for BSI. In patients receiving tandem HCT, the pathogens isolated from blood cultures were all gram-positive bacteria, with Streptococcus mitis/oralis accounting for up to 67% of the isolated pathogens. Tandem HCT and PICCs were identified as independent risk factors for BSI after allogeneic HCT in children/AYAs. The pathogens were commonly gram-positive, and Streptococcus mitis/oralis is important in patients who received tandem HCT. These data can provide valuable information for future studies to consider effective interventions to reduce the risk of BSI in high-risk patients.


Introduction
Allogeneic hematopoietic cell transplantation (HCT) has been used to treat various diseases, including hematological malignancies, solid tumors, and genetic diseases [1].Although HCT can cure these patients, it often leads to life-threatening side effects such as infections, graftversus-host disease, and organ damage.Infections is the most common serious complication that develops within the first few weeks after HCT.
Patients are treated with immunosuppressive drugs to support the engraftment of the donor's hematopoietic stem cells to the recipient, which leads to immune system suppression.Neutrophils, which play an important role in fighting against bacterial and fungal infections, are present in undetectable levels in the first few weeks after HCT until neutrophils are derived from the transplanted donor's hematopoietic stem cells.During this period, patients are highly susceptible to bacterial and fungal infections.Even after hematopoietic stem cells are engrafted and differentiated into various hematopoietic cells, patients are still susceptible to infection.In this phase, patients are more susceptible to viral or fungal infection than to bacterial infection, because the functions of lymphocytes are diminished owing to the use of immunosuppressive drugs to prevent graft-versus-host disease.
Bacterial and fungal infections can occur in any organ.Bloodstream infection (BSI), in which bacteria or other microorganisms enter the bloodstream and spread throughout the body, is a severe form of infection.It causes organ damage and occasionally leads to sepsis.The incidence of BSI after HCT ranges from 8.7% to 34% [2,3], and the majority of events occurred in the pre-engraftment phase [4][5][6].BSI after HCT is associated with high mortality rates [7,8], and it should be managed to perform a safe and effective HCT.The risk factors for BSI reported to date include the use of unrelated donors, umbilical cord blood, and myeloablative conditioning regimens, mucositis, and high-risk malignant diseases [2,[9][10][11][12].However, the incidence, risk factors, and associated pathogens of BSI vary widely by geographic location and patient population [2].To effectively manage BSI after HCT, we retrospectively investigated the responsible pathogens and risk factors of BSI after HCT in children/adolescents and young adults (AYAs) at our hospital.

Participants
This study included children/AYAs (< 31 years old) who received allogeneic HCT at the Department of Pediatrics, Nagoya University Hospital between January 1, 2018 and March 31, 2022.This study was approved by the ethics committee of Nagoya University Graduate School of Medicine (approval no.2022-0276).An opt-out consent process was approved by disseminating the research information; thus, the requirement of informed consent was waived.All research processes were conducted in accordance with the principles of the Declaration of Helsinki.

Data collection
Medical records of the patients were reviewed to retrieve data on the demographic characteristics of the participants; HCT details such as donor source, patient and donor human leukocyte antigen, conditioning regimen, supportive care, catheter insertion, and adverse events evaluated using the Common Terminology Criteria for Adverse Events v5.0; and information on BSI including onset time, isolated pathogens, and treatment outcomes.
Data were collected between October 24, 2022 and April 30, 2023.All personal data were coded to ensure that individuals could not be identified in the analysis.

Statistical analysis
The cumulative incidence of BSI was calculated using the competing risk method and compared using Gray's test.The subdistribution hazard ratios of BSI development were obtained using the Fine-Gray proportional hazards model.All tests were two-tailed.A type I error of <0.05 was considered statistically significant.All statistical analyses were performed using EZR (Saitama Medical Center, Jichi Medical University, Saitama, Japan) [13].

Definition
A definitive BSI was defined as the isolation of a pathogen from the blood of a patient with fever or other signs of infection.For common skin contaminants such as coagulase-negative staphylococci and Corynebacteria, BSI was confirmed only after two consecutive blood cultures were positive for the same bacterial strain [14].We also defined a probable BSI as a case in which only one set of blood cultures was obtained, common skin contaminants were detected in the blood culture within a short period (�72 hours), and the clinical picture was considered highly consistent with BSI.The current study aimed to analyze BSI that occurred within 30 days after allogeneic HCT.Conditioning regimens used before transplantation were classified as myeloablative and reduced-intensity regimens, as described by Bacigalupo et al. [15].Tandem HCT was defined as a planned allogeneic HCT after autologous HCT with highdose chemotherapy provided within 6-month intervals.

Characteristics of the patients
We performed 127 allogeneic HCTs for 117 consecutive patients aged <31 years at the Department of Pediatrics, Nagoya University Hospital, between January 1, 2018, and March 31, 2022.After excluding two patients who underwent six HCTs as a rescue treatment for primary graft failure (n = 4) and those who did not receive the conditioning regimen (n = 2), we included 115 consecutive patients who underwent 121 allogeneic HCTs in this retrospective study (S1

Prophylaxis and treatment of infection
All patients were treated at the hospital for at least 60 days.Polymyxin B sulfate (100,000 units/ kg/day), fluconazole (3 mg/kg/day), and valacyclovir (75 mg/kg/day for <10 kg, 50 mg/kg/day for �10 kg) were administered until 90 days after HCT to prevent infections.Sulfamethoxazole/trimethoprim (4-8 mg/kg/day as trimethoprim, 3 days/week) was also administered after engraftment and continued for 1 year after HCT.Within the first 90 days after allogeneic HCT, patients received intravenous immunoglobulin (100-200 mg/kg) once a week.  2 and Fig 2).Multidrug-resistant organisms were detected in only one case with multidrug-resistant Pseudomonas aeruginosa, for which the patient was a carrier prior to HCT.

Univariable and multivariable analyses of risk factors for BSI
The findings of the univariable and multivariable analyses of risk factors for BSI are summarized in Table 3.In the univariable analysis, the cumulative incidence of BSI was higher in patients receiving tandem HCT, reduced-intensity conditioning, human leukocyte antigenmismatched HCT, cord blood transplantation, or having malignant diseases (Table 3).In

PLOS ONE
Bloodstream infection risk factors in children/AYA hematopoietic cell transplantation recipients

Discussion
In this retrospective study, the clinical features and risk factors for BSI in the first 30 days after allogeneic HCT in a cohort of children/AYAs at a single institution were analyzed.From 0 to 30 days after HCT, 27 of 121 patients developed BSI, and tandem HCT and PICCs were identified as independent risk factors for BSI.The use of PICC catheters is increasing with the development of novel devices.In addition, based on relatively recent evidence, tandem HCT has become widely used, especially in pediatric neuroblastoma cases [16,17].This is unique because there are no reports that have identified PICC and tandem HCT (a relatively new clinical factor in pediatric HCT) as a possible risk factor for BSI.It is unclear why tandem HCT was found as a risk factor for early post-HCT BSI in the current cohort.All tandem transplant recipients in this cohort (n = 21) underwent allogeneic cord blood transplant after autologous transplant to improve outcomes in high-risk neuroblastoma, but do not include cases enrolled in ongoing prospective trials (jRCTs041210034).Recent studies have reported that pre-transplant oral/intestinal microbiome profiles are strongly associated with the development of serious infections early after HCT [18,19].Another study reported that the diversity of the oral microbiome considerably decreased after HCT; however, it was restored to its pre-HCT composition by 3 months [20,21].Future studies should be conducted to assess the impact of microbiome changes on the infectious complications of tandem HCT in patients receiving the initial HCT.
In this study, 89% of the pathogens isolated from patients were gram-positive bacteria, and only one case (4%) had gram-negative bacteria.Gut decontamination with polymyxin B in patients with HCT has been a long-standing institutional practice at Nagoya University Hospital due to its broad gram negative coverage and lack of systemic absorption.This clinical practice may have contributed to a reduction in the rate of gram-negative bacterial infections in this cohort.In addtion, there was also a trend toward a lower incidence of BSI in patients who received intravenous antibiotics on day 0 of HCT (HR 0.46 [0.21-1.01],p = 0.054).The routine use of prophylactic antibiotics in pediatric patients undergoing HCT is not recommended by international guidelines [22][23][24].Levofloxacin is widely used to prevent bacteremia in adult patients undergoing HCT.However, data supporting the effectiveness of this approach in children are inconsistent [3].Recently, Gardner et al. [25] reported the effectiveness of levofloxacin in pediatric and adult patients receiving HCT in reducing the incidence of bacteremia.However, it is associated with long-term toxicity to the musculoskeletal system in children [26].A recent study found that pediatric patients with HCT who received intravenous prophylactic antibiotics (cefepime or piperacillin/tazobactam) had an extremely low incidence of BSI (3%) but a high incidence of Clostridioides difficile infection (10%) [27].Hence, future prospective studies should be performed to evaluate the use of prophylactic antibiotics in specific patient groups at high risk for BSI, such as pediatric patients receiving tandem HCT.
PICC is a non-tunneled central venous catheter (CVC) inserted via the peripheral vein in the upper arm.This catheter is widely used in pediatric patients with hematologic and neoplastic diseases because of its more convenient insertion and removal than the tunneled CVCs.In this study, PICC was considered a risk factor of BSI after allogeneic HCT (Table 3), particularly in non-tandem HCT (S6 Table ).Several studies have investigated the incidence of BSI in pediatric patients with PICCs and tunneled CVCs.Nevertheless, it is unclear whether PICCs are a risk factor for BSI [28][29][30][31][32].The types of CVC devices appropriate for pediatric patients undergoing HCT should be evaluated in larger patient populations.
There are several limitations of this study.First, this was a single-center, retrospective study of a relatively small number of patients, and the timing and type of antibiotics used were not strictly standardized.In addition, the results should be interpreted with caution because of the relatively small number of events (n = 27) in the overall cohort, which reduces the power of statistical analyses.Second, because this cohort consists of pediatric patients, we have included cases in which the standard two sets of blood cultures were not performed due to difficulties with venipuncture.There were four cases in which common skin contaminants were positive for blood culture (Staphylococcus epidermidis: 3, Staphylococcus haemolyticus: 1), but only one case in which two sets of blood cultures were obtained and both were positive.However, the remaining three cases had positive blood cultures within a short period of time (72 hours or less), and their clinical presentation indicated a high probability that they were true BSIs, so they were treated as "probable BSIs" in this study.Third, this retrospective analysis precluded a combined analysis of the oral/gut microbiome, which has recently been implicated in post-HCT infectious complications.Future multicenter prospective studies should be conducted to provide a more accurate image of the pathogenesis, biomarkers, and risk factors of BSI in patients who received HCT.
In conclusion, this study identified tandem HCT and PICCs as independent risk factors for BSI in the early phase after allogeneic HCT in children/AYAs.The pathogens were commonly gram-positive, and Streptococcus mitis/oralis is important in patients who received tandem HCT.These data can provide important information for future studies investigating the interventions considered effective in reducing the risk of BSI in high-risk patients.

Fig 2 .
Fig 2. The relationship between pathogens isolated from blood cultures and the onset of bloodstream infection.The number of patients based on the onset of bloodstream infection is indicated by a different color for each pathogen isolated.The asterisk (*) indicates probable BSI patients.Abbreviations: HCT, hematopoietic cell transplantation.https://doi.org/10.1371/journal.pone.0308395.g002

Table 3 and Fig 1B and 1C). Subgroup analysis of BSIs in patients undergoing tandem HCT S3 Table shows
the clinical and transplant characteristics of 21 patients who underwent tandem HCT.In total, 12 of 21 patients developed BSI, with a cumulative incidence of BSI at 30 days after HCT of 57.1% (95% CI: 32.9%-75.5%;Fig1B).All pathogens isolated from blood cultures were gram-positive bacteria, with Streptococcus mitis/oralis, accounting for 67% (S2

Table ) ;
83% of BSI cases developed <10 days after HCT (S2A Fig).Univariable analysis did not identify other factors associated with the cumulative incidence of BSI (S4 Table).

Subgroup analysis of patients receiving non-tandem HCT S5 Table depicts
the clinical and transplant characteristics of 100 patients who underwent non-tandem HCT.A total of 15 patients developed BSI, with a cumulative incidence of BSI at 30 days after HCT of 15.0% (95% CI: 8.8%-22.7%;Fig1B).Although the bacterial species varied between patients who received tandem and non-tandem HCT, 13 (81%) of 16 pathogens isolated from blood cultures of patients non-tandem HCT were gram-positive organisms (S2